This invention relates to a printer, particularly to a thermal printer adapted to print characters or a picture by generating a heat from a thermal head.
Usually, a thermal printer has a thermal head which comprises a large number of exothermic resistors arranged on an insulating base. By selectively supplying electric current to the exothermic resistors, a Joule heat is generated which is applied to a thermosensible paper or an ink film so as to print characters or a picture.
FIG. 7 shows the basic construction of a conventional thermal printer comprising a drum 3, an ink film 4 and a thermal head 2. As illustrated in FIG. 7, a paper 1 is clamped to the drum 3 using a clamping means (not shown) so as to be positioned between the drum 3 and the ink film 4. The thermal head 2 is caused to press the ink film 4 against the paper 1, thereby printing characters or pictures on the paper 1 using the heat from the thermal head 2.
FIG. 8 shows in detail an example of the thermal head 2. Referring to FIG. 8, the thermal head 2 comprises an aluminum base 5. Provided under the aluminum base 5 is a ceramic base 6 having a number of exothermic members, a substrate member 7 for exothermic member driving circuit, a protection cover 8 for covering the substrate member 7. Further, a plurality of heat radiation fins 11 are integrally formed on the aluminum base 5.
FIG. 9 shows in detail an example of a clamping means 20 generally used in the above conventional thermal printer. Referring to FIG. 9, the clamping means 20 has a plurality of pressing members 21 which are provided so as to easily get contact with the drum 3 or leave therefrom. In use, the paper 1 is fed inwardly until it gets to a stopper member 22, thus the front portion of the paper 1 will be clamped to the drum 3 by virtue of the pressing members 21. Then, with the proceeding of the operation, the drum 3 is rotated to cause the paper 1 to be rolled gradually around the outer periphery of the drum 3. Further, with the use of pressing rollers 23, the paper 1 is pressed against the drum 3 so that it gets tight contact with the outer periphery of the drum 3.
When the printing operation is performed, the thermal head 2 is caused to press the ink film 4 against the paper 1, thereby printing characters or pictures on the paper 1, using the heat from the thermal head 2. During the printing operation, the drum 3 is rotated at a speed corresponding to a printing speed of the thermal head 2, whilst the ink film 4 is moved at a speed corresponding to the drum speed.
The ink film 4 includes four color areas (for example, yellow, magenta, cyanogen, black) which are arranged successively and in a manner such that the four color areas appear repeatedly on the ink film 4 along the entire length thereof. As soon as the printing of a first color (yellow) is finished, the drum 3 is rotated to return to its home position, so that the front portion of the same paper 1 is caused to be coincident with that of a second color (magenta) area of the ink film 4. In the same manner, a third color (cyanogen) and a fourth color (black) may be printed on the paper 1 so as to obtain a completed color picture.
The thermal head 2 used in the above thermal printer is pivotably supported on the printer cover member through appropriate bracket means, such that the thermal head 2. can easily get contact with the drum 3 and easily leave therefrom. When the paper 1 is being clamped around the drum 3 or is being discharged therefrom, the thermal head 2 can leave from the drum 3 when the drum rotates, so that the thermal head 2 is prevented from any possible damage due to a friction with the drum 3.
However, in the above thermal printer, since it is impossible for the thermal head 2 to absorb a possible thickness difference in a single piece of paper, the thermal head 2 fails to get uniform contact with the paper 1, resulting in a problem such as irregular color printing.
Moreover, in the above thermal printer, since the thermal head 2 receives a vibration from the drum 3 while the drum rotates, the relative position of the thermal head 2 with respect to the drum 3 will gradually change with the passing of time, rendering the thermal printer to be unreliable in actual use.